1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
There are some power conversion circuits which accomplish higher efficiencies by implementing a mechanism that accomplishes switching at zero voltage. Power loss in a switch is the product of the voltage applied across the switch and the current flowing through the switch. In a switching power converter, when the switch is in the on state, the voltage across the switch is zero, so the power loss is zero. When the switch is in the off state, the power loss is zero, because the current through the switch is zero. During the transition from on to off, and vice versa, power losses can occur, if there is no mechanism to switch at zero voltage or zero current. During the switching transitions, energy losses will occur if there is simultaneously (1) non-zero voltage applied across the switch and (2) non-zero current flowing through the switch. The energy lost in each switching transition is equal to the time integral of the product of switch voltage and switch current. The power losses associated with the switching transitions will be the product of the energy lost per transition and the switching frequency. The power losses that occur because of these transitions are referred to as switching losses by those people who are skilled in the art of switching power converter design. In zero voltage switching converters the zero voltage turn off transition is accomplished by turning off a switch in parallel with a capacitor and a diode when the capacitor's voltage is zero. The capacitor maintains the applied voltage at zero across the switch as the current through the switch falls to zero. In the zero voltage transition the current in the switch is transferred to the parallel capacitor as the switch turns off.
The zero voltage turn on transition is accomplished by discharging the parallel capacitor using the energy stored in a magnetic circuit element, such as an inductor or transformer, and turning on the switch after the parallel diode has begun to conduct. During the turn on transition the voltage across the switch is held at zero, clamped by the parallel diode. The various zero voltage switching (ZVS) techniques differ in the control and modulation schemes used to accomplish regulation, in the energy storage mechanisms used to accomplish the zero voltage turn on transition, and in a few cases on some unique switch timing mechanisms.
One of the ZVS techniques uses an inductor or transformer with relatively low inductance so that the inductor current reverses sign during each switching cycle. An example of a buck converter with this property is shown in FIG. 1 and its wave forms are illustrated in FIG. 2. One advantage of this technique is that the switching transitions are all zero voltage transitions driven by the stored energy and current in the inductor. Another advantage is that the inductor can be made small and the inductance needs to be small in order that the current can be reversed during each switching cycle. The disadvantages are that the output current reverses each cycle so that the output capacitor must be relatively large and must store a substantial amount of energy and be able to accommodate the large ripple currents. Although the inductor can be made smaller because the inductance is reduced, the size reduction of the inductor is not as large as might be suggested by the reduction in inductance value. In a typical hard switching buck converter the output choke would be saturation limited. Its core losses would be small by comparison to its copper losses. With a small value inductor with large current swings the inductor will more likely be core loss limited, so that the cross section, the core gap, and the number of turns would need to be increased to reduce the flux swing and associated core losses. Also, in the typical hard switching buck converter in which the inductor current has a large DC component and a small AC component the AC copper winding losses are typically very small. In the FIG. 1 circuit the issue of AC winding losses must be addressed by suitable magnetic circuit element design (Litz wire or properly placed and oriented copper foil or strip) or AC winding losses will be substantial. Another disadvantage of the small inductance value technique is that there will be much higher peak currents in the choke winding and in the switches which will result in additional conduction losses in those elements. Another disadvantage of the small inductance value technique is that the energy and current available to drive the zero voltage transitions decreases as the load current increases so that in an over load condition there may be no energy available to drive a zero voltage transition and there may be substantial switching losses at the same time that the conduction losses are at their highest levels. In general, almost any power converter can be made to have zero voltage switching by this mechanism. That is, almost any power converter can be designed so that the current in its principal magnetic circuit element(s) reverses each cycle so that the stored energy in its magnetic storage element(s) is directed in a way which will enable a zero voltage transition on every switching transition.